As devices for measuring a thickness of a film formed on a substrate, there exist an ellipsometer (Patent Document 1, for example) and a measuring device in which a film thickness is obtained from the wavelength which shows maximum or minimum in spectral reflectivity (which is hereinafter referred to as a PV (Peak-Valley) device) (Patent Document 2, for example).
The ellipsometer is widely used for measurement of a thin film thickness in the field of manufacturing semiconductors. However, the ellipsometer has the following problems. Since the projection angle and the receiving angle are large, the ellipsometer can hardly be used in a process in which a distance to the object will change. Since optical elements both on the projection and receiving sides have to be rotated for measurement, the optical system is complicated and expensive.
Since, in the PV device, a film thickness is obtained from the wavelength which shows maximum or minimum in spectral reflectivity, it is necessary that the wavelength which shows maximum or minimum in spectral reflectivity exists. However, in general, in spectral reflectivity data of thin films, thickness of which is 500 nm or less, the wavelength which clearly shows maximum or minimum will not exist. Accordingly, the conventional PV device cannot be used for measurement of thin films, thickness of which is 500 nm or less.
Under the situation, the inventors of the present application and others have developed a device and a method which use theoretical values of characteristic variable of color in order to measure thickness of thin films which is 500 nm or less (Patent Document 3). In this method, however, refractive index of a film is determined based on an estimated value of thickness of a sample film, and therefore the application area is limited.
Thus, a film thickness device of a simple structure and a film thickness method which can be widely used for measurement of thickness of thin films including those, thickness of which is 500 nm or less have not been developed.
Thus, there is a need for a film thickness measuring device of a simple structure and a film thickness measuring method which can be widely used for measurement of thickness of thin films including those, thickness of which is 500 nm or less.